Mymetrics LidR canopy cover

Question

I am trying to compute my own metric using LidR. I would like to compute the canopy cover metric as (# of first returns > 2m) / (total number of first returns)

The package suggested this function from the Examples to compute this metric

myMetrics = function(z,rn){
  first  = rn == 1L
  zfirst = z[first]
  nfirst = length(zfirst)
  above2 = sum(z> 2)
  above2
  x =(above2/nfirst)*100
  x
  # User's metrics
  metrics = list(
    above2aboven1st = x, # Num of returns above 2 divided by num of 1st returns
    zsqmean = sqrt(mean(z^2))  # Quadratic mean of z
  )
  metrics
  # Combined with standard metrics
  return( c(stdmetrics_z(z),metrics))
}

metrics = grid_metrics(las, ~myMetrics(Z, rn=ReturnNumber))

However, the results of the metric using this function does not work correctly, since my values range between 0 and 300

it would be possible to calculate a density metric in height intervals as in FUSION or Lastools?

e.g number of all returns between 0.2 and 3 meters/number of total returns in 20 meters cell

Answer 1

You are not computing (# of first returns > 2m) / (total number of first returns) but instead (# of returns > 2m) / (total number of first returns). Did you get this for the documentation. If yes it is a mistake in the doc.

myMetrics = function(z,rn){
  first  = rn == 1L
  zfirst = z[first]
  nfirst = length(zfirst)
  firstabove2 = sum(zfirst > 2)
  x = (firstabove2/nfirst)*100
  metrics = list(
    above2aboven1st = x, # Num of returns above 2 divided by num of 1st returns
    zsqmean = sqrt(mean(z^2))  # Quadratic mean of z
  )
  metrics
  # Combined with standard metrics
  return( c(stdmetrics_z(z),metrics))
}